The present invention relates to a rear wheel steering method for a vehicle of the type steering rear wheels when front wheels are steered by a steering wheel and, more particularly, to a method of steering rear wheels in a low vehicle speed range in which front and rear wheels are steered in opposite directions to each other.
Some modern vehicles are loaded with a four wheel steering system in which rear wheels are steered together with front wheels. Specifically, when the operator of this kind of vehicle steers the front wheels by a certain angle via the steering wheel, the steering system steers the rear wheels by a predetermined steering ratio matching the instantaneous vehicle speed. In the four wheel steerage vehicle, steering the front and rear wheels in opposite directions to each other in a low vehicle speed range is successful in enhancing the small turn capability and, therefore, the manageability of the vehicle. On the other hand, steering the front and rear wheels in the same directions in a high speed range is successful in promoting the ease of steering operations and stability. Therefore, the steering ratio of the front and rear wheels of a vehicle of the type concerned is usually set such that the front and rear wheels are steered in opposite phases in a low vehicle speed range and in the same phase in a high vehicle speed range. Further, the steering ratio is varied smoothly in matching relation to the varying vehicle speed in order to prevent the steering angle from being sharply changed between the high and low speed ranges.
It has been customary with the above-described type of vehicle to reduce, assuming that the steering ratio which gives the front and rear wheels the opposite phases is positive, the steering ratio sequentially with the increase in vehicle speed, as disclosed in Japanese Patent Laid-Open Publication 11173/1982. For example, assume that the vehicle is started with the front wheels thereof being held in a certain angular position. Then, the rear wheels are steered by the greatest angle in the opposite direction to the front wheels when the vehicle speed is zero, and the steering angle of the rear wheels decreases with the increase in vehicle speed. On the acceleration of the vehicle to a speed of 35 kilometers per hour or so, the steering angle of the rear wheels is reduced to zero. As the vehicle is further accelerated, the rear wheels are steered in the same direction as the front wheels with the steering angle thereof being increased little by little.
Generally, a vehicle shows a tendency to understeer, i.e., the radius of a turn increases with the increase in vehicle speed when the vehicle is accelerated while being steered by a certain angle. Although this tendency is not noticeable in a low vehicle speed range such as just after the start of the vehicle, it grows noticeable little by little as the vehicle speed increases to about 10 kilometers per hour. Therefore, from the manageability standpoint, it is preferable that the radius of a turn be maintained small against the tendency to understeer by steering the rear wheels by a great angle. With the conventional scheme which reduces the steering ratio monotonously with the increase in vehicle speed, it is necessary to provide the steering ratio with a greater maximum value, i.e., to set a sufficiently great steering ratio in a very low vehicle speed range. However, the increase in steering ratio directly translates into the increase in the change of the radius of a turn relative to the operation of the steering wheel. Furthermore, one often starts a vehicle from a standstill by turning the steering wheel by a great angle. In such a case, should the rear wheels be steered greatly in the opposite direction to the front wheels, the locus of the rear wheels would project to the outside. For this reason, the steering ratio should not be increased beyond a certain value in the very low vehicle speed range just after the start of the vehicle.
Since the steering ratio available in the very low vehicle speed range is limited as stated above, the prior art system which reduces the steering ratio in a range higher than the very low vehicle speed range cannot suppress the increase in the radius of a turn ascribable to acceleration and, therefore, fails to implement sufficient manageability. It has been proposed to maintain the steering ratio at a constant maximum value until the vehicle speed increases to a certain degree. This, however, cannot improve the manageability of the vehicle satisfactorily because the maximum steering ratio is limited.
In the light of the above, the steering ratio of the rear wheels to the front wheels may be maintained small until the vehicle travels a predetermined distance after the start from a standstill, as taught in Japanese Patent Laid-Open Publication 176867/1985. Then, even when the steering wheel is turned by a great angle immediately after the start of the vehicle, the steering angle of the rear wheels will be maintained small to prevent the rear portion of the vehicle from projecting to the outside. As the vehicle travels by the predetermined distance to a position where the above-mentioned projection does not matter, the rear wheels are steered by a predetermined steering ratio. Hence, if this steering ratio is selected to have a relatively large value, sharp small turns will be insured thereafter. However, the problem with this scheme is that since the steering angle of the rear wheels cannot reach a predetermined value until the vehicle runs the predetermined distance, the small turn capability of the vehicle is degraded for some time after the start. Specifically, when the operator parked the vehicle starts it by turning the steering wheel and then turns the steering wheel in the opposite direction by a great angle, the vehicle cannot make a small turn. This is undesirable from the manageability standpoint. Another problem is that when the vehicle is steered while being started relatively rapidly, the radius of a turn cannot be prevented from increasing. In addition, although the vehicle may be operated at a constant speed by a constant angle of the steering wheel, the yaw rate suddenly changes when the predetermined distance is reached.